Generally, in the production of various products utilizing lactic bacteria such as fermented milk, a method for controlling a fermentation of the culture, namely, a method for investigating the state of the fermentation proceeded and determining the completion of the fermentation, is carried out by employing the concentration of a saccharide as a substrate, the concentration of lactic acid as a product formed following fermentation, and values of the acidity and the pH value of a fermented liquid, as indexes in the control of the step of culture. In order to obtain these values, conventionally a fermented liquid has been sampled once at a proper step, and then have been analyzed quantitatively regarding each item according to an ordinary procedure, utilizing, for example, a neutralization titration method. According to such a method, however, there occurs a problem of a large accidental error in titration for measuring each index value; in addition, since much time and many hands are required, it has not been possible to control the step of cultivation in a simple and rapid manner.
Moreover, recently there have been found examples investigating a possibility of a method of more reliable and simple on-line measurement; however, they are only for obtaining basic knowledge about on-line measurement in the step of the fermentation of specific products and are still matters of basic investigation; and no example actually investigating whether it is possible or not to control the step of cultivation of lactic bacteria according to such methods of measurement has been reported, and hence it has been desired keenly in the field concerned to develop a new method for controlling the step of cultivation of lactic bacteria utilizing a method of more reliable and simple measurement.
As described above, it has been desired keenly to develop a method for measuring the concentration of a saccharide, the concentration of lactic acid, acidity and a pH value as indexes for controlling the state of culture of lactic bacteria in a rapid, simple and precise manner, particularly, to develop a method of in-line measurement and a method for controlling culture of lactic bacteria based thereon.
The present inventors have engaged in assiduous studies with a view to developing such a new method for controlling culture of lactic bacteria, and as a result have found that the infrared absorption spectrum of a fermented liquid is measured by the FT-IR spectrometry (Fourier transform infrared spectroscopy) and that of the infrared absorption assignable to dissociable lactic acid, the intensity of the infrared absorption assignable to nondissociable lactic acid in the culture, the intensity of the infrared absorption assignable to an alcoholic C-O group of glucose and the intensity of the infrared absorption assignable to an alcoholic C-O group of lactose are measured, and the lactic acid concentration, the pH value, the glucose concentration and the lactose concentration of the culture can be measured based on these measurements and that the state of culture of lactic bacteria can be monitored and controlled in a simple and rapid manner, by employing these measurements as indexes, which has led to the completion of the present invention. In addition, they have found that, of FT-IR methods, the ATR-FT-IR spectrometry, a method for measuring a whole reflective spectrum, enables the numerical values of various indexes to be measured and, based on the measurements, the culture of lactic bacteria is automatically controlled.